Measuring Primary Production Using 14C Radiolabeling

What is Primary Productivity?

Primary productivity is the process by which organisms make their own food from inorganic sources. The majority of primary producers are terrestrial plants and microbial life, such as algae. These organisms are known as autotrophs, since they can use inorganic substrates and solar energy to carry out metabolic processes and build cellular material. Primary productivity due to photosynthesis is commonly measured by quantifying oxygen production or CO2 assimilation.

How Does This Method Work?

The theory behind using 14C to measure productivity involves using a labeled tracer to quantify assimilated carbon. The 14C method estimates the uptake and assimilation of dissolved inorganic carbon (DIC) by planktonic algae in the water column. The method is based on the assumption that biological uptake of 14C-labelled DIC is proportional to the biological uptake of the more commonly found 12C DIC. In order to determine uptake, one must know the concentration of DIC naturally occurring in the sample water, the amount of 14C-DIC added, and the amount of 14C retained in particulate matter (14C-POC) at the end of the incubation experiment. A 5% metabolic discrimination factor may be applied to the data as well, since organisms preferentially take up lighter isotopes. Carbon uptake can be measured by the following equation:

C uptake = (naturally occurring DIC x 14C-POC x 1.05)/(14C-DIC added)

Applications

Aquatic primary productivity generally governs the biological activity of a lake. Since primary productivity makes up the bottom-most trophic level, it provides essential nutrients and energy to higher trophic levels and higher organisms. For instance, primary productivity in a lake may supply oxygen to aerobic organisms such as insects and fish, and so times of stressed productivity may result in a decline in the fish population. Low primary productivity may thus hinder other lake biota by limiting available nutrients. Conversely, extremely high primary productivity may result in algal blooms, which may eventually lead to mass kills at other trophic levels due to nutrient depletion or by high turbidity from massive concentrations of plankton. So, relatively high rates of primary productivity may support the most diverse and largest amount of biomass, but productivity can also become too high for a system to handle.

How To- Sample Collection, Protocol, Analysis, and Considerations

Protocols and Analyses:

There are several protocols for measuring rates of primary productivity via 14C. The basic idea remains the same: before dawn, water samples are collected in a series of bottles- generally two clear and one amber/dark colored bottle through which light is not able to penetrate. The water samples are inoculated with 14C, capped, and placed in the environment they were collected from for a day (this time may vary). Following the incubation, the samples are collected under low light conditions and filtered through a 0.2um filter. The filter is placed into a liquid scintillation vial, acidified to purge excess 14C, and kept cold until it can be analyzed in a scintillation counter.

The following resources provide step-by-step protocols for measuring primary productivity using 14C.

Considerations:
The 14C method for measuring productivity has several important considerations, including:

This method assumes that 14C will be taken up proportionally to the more naturally prevalent 12C species.

Bottles should be inoculated and collected in low-light conditions (pre-dawn or post sunset) in order to ensure that samples collected from subsurface waters will not receive sunlight they would not naturally receive.

Samples should be filtered as soon as possible after collection. Excess (non-incorporated) 14C can be purged from the filter using a small amount of 1M HCl (this will drive off excess 14C as CO2.

When running a scintillation count, dissolving the filter in the scintillation cocktail is not necessarily bad, since it ensures that all 14C will be in solution and thus be counted.

Results Analysis

Availability of Photosynthetically Active Radiation (PAR), or available light. This can be influenced by lake depth, turbidity/suspended sediment load, and shading by macrophytes, or plants

Biota dynamics in the system (e.g. population composition, number of primary producing organisms, competition, population establishment time vs. disturbance)

Rates of primary productivity can range from 0-9000 kcal/m2/yr, with desert regions having lower rates and estuaries having higher rates (see The Flow of Energy: Primary Production to Higher Trophic Levels for more details). More constrained rates of primary productivity for specific environments may be researched by looking at primary literature for a specific site.